Influence of hydrogen on formation and stability of Zr-based quasicrystals

Hydrogenation of quasicrystalline Zr_69.5Cu₁₂Ni₁₁Al_7.5 was performed electrochemically in a 2:1 glycerine-phosphoric acid electrolyte. Hydrogen absorption/desorption as well as the influence of hydrogen on the formation and stability was studied by means of X-ray diffraction (XRD), transmission electron microscopy (TEM) and thermal desorption spectroscopy (TDS). In comparison to amorphous Zr-Cu-Ni-Al the absorption kinetics in quasicrystalline Zr-Cu-Ni-Al were found to be faster. Desorption is hindered in both materials probably due to the formation of thin ZrO₂ layers. Only partial desorption of hydrogen was observed by means of TDS to occur prior to the decomposition of the quasicrystalline phase. The observed increase in length during hydrogen charging indicates an interesting new result. Instead of the continuous increase of the specific volume per hydrogen atom at very low hydrogen concentration typical for the filling up of larger trapping sites, a decrease from a rather high value is observed. This means that not only the site for the hydrogen atom is expanded, but also the neighboring still empty ones. Above a hydrogen content of H/M = 0.05 the formation of Zr-Cu-Ni-Al quasicrystals is replaced by tetragonal Zr₂Cu, tetragonal Zr₂Ni and hexagonal Zr₆NiAl₂. At high hydrogen contents (about H/M = 1.0) phase separation is assumed to take place followed by the formation of nanocrystalline ZrH₂. Icosahedral Zr_69.5Cu₁₂Ni₁₁Al_7.5 was found to decompose through a discontinuous transformation by complex precipitation reactions; it transforms mainly into tetragonal Zr₂Cu, tetragonal Zr₂Ni and hexagonal Zr₆NiAl₂. Hydrogenation of quasicrystals at low concentrations does not change the phases formed during decomposition. At hydrogen contents higher than H/M = 1.1 decomposition of quasicrystals starts with the precipitation of tetragonal ZrH_2-x.